This invention relates to magnetic recording media such as tapes and discs which are obtained by applying a magnetic coating on a non-magnetic support.
General purpose magnetic tapes and discs are produced by coating a polyethylene terephthalate film with a magnetic material prepared by dispersing ferromagnetic particles about 1 micron long in a resinous binder. The resinous binder plays a very important role in providing excellent dispersibility, filling efficiency, and orientation of magnetic particles as well as imparting excellent durability, abrasion resistance, heat resistance and smoothness to the magnetic coating and adhesion thereof to the support.
Examples of resinous binders conventionally used include vinyl chloride/vinyl acetate copolymers, vinyl chloride/vinyl acetate/vinyl alcohol copolymers, vinyl chloride/vinylidene chloride copolymers, polyurethane resins, polyester resins, acrylonitrile/butadiene copolymers, nitrocellulose, cellulose acetate butyrate, epoxy resins, and acrylic resins. Of these resins, conventional polyurethane resins have excellent toughness and abrasion resistance compared to other resins but often are inferior in properties such as blocking resistance, heat resistance, and running stability. For these reasons, a mixed system of polyurethane resins with nitrocellulose or vinyl chloride/vinyl acetate copolymers is often used. The durability, heat resistance, and adhesive properties of polyurethanes may be improved by curing with a polyisocyanate at from about room temperature to about 40.degree. C. or higher after the application and drying of the magnetic coating composition.
A magnetic recording layer having highly improved strength and other properties employs a binder resin comprising both a vinyl chloride copolymer (e.g., a vinyl chloride/vinyl acetate/maleic anhydride copolymer) and a polyurethane resin. Japanese Patent Publication No. 59-8127 teaches the incorporation of a polar group into one or both of the constituent resins to enhance the dispersibility of ferromagnetic powders in such a binder.
The durability and abrasion resistance of conventional resinous binders are still insufficient for use in video tapes, computer tapes, and floppy discs, all of which are required to have high performance and high reliability. Demand for high density and high quality recording media is increasing while smoothness is still desired. As the required smoothness increases, the running durability has suffered and resinous binders with higher durability must be developed. To do so, it has been proposed to introduce multifunctional components into the polyurethanes which are reactive with the polyisocyanate; trimethylolpropane and diethanolamine exemplify such components. A serious drawback to this approach is that the dispersibility of the magnetic particles often decreases as the durability improves. The high recording density and high quality required for magnetic media have been supplied in recent years by fine magnetic particles of metals and barium ferrite but durability and dispersibility are still required of resinous binders for such particles.
A method for improving the dispersibility of the particles by the incorporation of metal sulfonate groups or metal salts of acidic phosphorus compounds is taught in Japanese Patent Publication Nos. 57-3134 and 58-41564 and in Japanese Patent Publication (Kokai) No. 61-48122. More recently, Yatsuka et al has taught in U.S. Pat. No. 5,009,960 that the presence of such multifunctional components in the resin containing the metal sulfonate group or metal salt of an acidic phosphorus compound for the purpose of improving the durability of such coating still results in a lesser dispersibility. Yatsuka et al further taught that the incorporation of a bicyclic amide acetal into the polyurethane resin will overcome the deficiencies of the prior art. A preferred polyurethane contains, as a functional group, a metal salt of an acidic phosphorus compound having the formula: ##STR2## wherein M is an alkali metal atom; R.sup.1 is a hydrocarbon group of 3 to 10 carbon atoms, X is an ester forming functional group, R.sup.2 is an alkyl group of 1 to 12 carbon atoms, a cycloalkyl group of 6 to 12 carbon atoms, an aryl group of 6 to 12 carbon atoms which may contain a halogen atom, a hydroxyl group, an amino group, or an OM' group wherein M' is a metal atom.